1. Field of the Invention
The present invention relates to a telephone terminal, a telephone system using this telephone terminal and a control method of the telephone terminal capable of connecting to a local area network (LAN) transmitting, e.g., a voice packet.
2. Description of the Related Art
In recent years, a system, enabling communications among Internet protocol (IP) telephone terminals and between the IP telephone terminal and an external communication network such as a public network, by connecting the IP telephone terminals equipped with a communication processing function and a media information processing function to the LAN, connecting the LAN to the external communication network via a main device and conducting protocol conversion and data format conversion through the IP telephone terminal and the main device, has been proposed.
By the way, in such a kind of system, a signal creep is added to a voice signal when the IP telephone terminal is connected to an analog line or a private line. When the IP telephone terminal makes a loud speakerphone communication in a room with a high level acoustic echo, a handset on a communication partner hears an anxious echo.
In the case of the IP telephone terminal, further, sometimes occurs a delay resulted from a network, which seldom occurs in the case of a conventional telephone set. When the delay occurs, the voice which is talked though a partner side handset creeps on a speakerphone side as the acoustic echo and an abnormal noise and an echo sound are heard during talking and when it is terminated on the handset side and harsh sounds are caused. In particular, an anxious degree tends to become increased in proportion to a length of a delay amount, so that when the delay amount becomes large, an echo amount should be further reduced.
Therefore, it is considered to provide an echo canceller function for deleting the acoustic echo to be added to the voice signal to the main device (for example, U.S. Pat. No. 6,404,887, B1).
In the meanwhile, the above-mentioned echo canceller function effectively operates in the case of connection between the IP telephone terminal and an office lime, but in the case of mutual connection of IP telephone terminals with less affects by creeping (peer-to-peer), the function becomes unstable inversely.